Partial encapsulation of stents

ABSTRACT

A method of making an implantable medical device includes extruding a first ePTFE tube and a second ePTFE tube, cutting a plurality of slits in the first ePTFE tube, positioning a radially expandable support layer between the first and second ePTFE tubes so that the slits span portions of the support layer, and laminating the first ePTFE tube to the second ePTFE tube through openings in the support layer.

PRIORITY

This application is a division of U.S. patent application Ser. No.12/538,361, filed Aug. 10, 2009, which is a division of U.S. patentapplication Ser. No. 10/873,062, filed Jun. 21, 2004, now abandoned,which is a division of U.S. patent application Ser. No. 09/848,740,filed May 3, 2001, now U.S. Pat. No. 6,770,087, which is a division ofU.S. patent application Ser. No. 09/388,496, filed Sep. 2, 1999, nowU.S. Pat. No. 6,398,803, which claims the benefit of U.S. ProvisionalPatent Application No. 60/118,269, filed Feb. 2, 1999. This applicationexpressly incorporates by reference the entirety of each of theabove-mentioned applications as if fully set forth herein.

BACKGROUND

The present invention relates generally to the field of medical devices,and more particularly, to encapsulation of stents.

Stents and related endoluminal devices are currently used by medicalpractitioners to treat portions of the vascular system that become sonarrowed that blood flow is restricted. Stents are tubular structures,usually of metal, which are radially expandable to hold a narrowed bloodvessel in an open configuration. Such narrowing (stenosis) occurs, forexample, as a result of the disease process known as arteriosclerosis.Angioplasty of a coronary artery to correct arteriosclerosis maystimulate excess tissue proliferation which then blocks (restenosis) thenewly reopened vessel. While stents are most often used to “prop open”blood vessels, they can also be used to reinforce collapsed or narrowedtubular structures in the respiratory system, the reproductive system,biliary ducts or any other tubular body structure. However, stents aregenerally mesh-like so that endothelial and other cells can grow throughthe openings resulting in restenosis of the vessel.

Polytetrafluoroethylene (PTFE) has proven unusually advantageous as amaterial from which to fabricate blood vessel grafts or prostheses usedto replace damaged or diseased vessels. This is partially because PTFEis extremely biocompatible causing little or no immunogenic reactionwhen placed within the human body. This is also because in its preferredform, expanded PTFE (ePTFE), the material is light and porous and ispotentially colonized by living cells becoming a permanent part of thebody. The process of making ePTFE of vascular graft grade is well knownto one of ordinary skill in the art. Suffice it to say that the criticalstep in this process is the expansion of PTFE into ePTFE followingextrusion from a paste of crystalline PTFE particles. Expansionrepresents a controlled longitudinal stretching in which the PTFE isstretched up to several hundred percent of its original length. Duringthe expansion process fibrils of PTFE are drawn out of aggregated PTFEparticle (nodes), thereby creating a porous structure.

If stents could be enclosed in ePTFE, cellular infiltration could belimited, hopefully preventing or limiting restenosis. Early attempts toproduce a stent enshrouded with ePTFE focused around use of adhesives orphysical attachment such as suturing. However, such methods are far fromideal, and suturing, in particular, is very labor intensive. Morerecently, methods have been developed for encapsulating a stent betweentwo tubular ePTFE members whereby the ePTFE of one-member contacts andbonds to the ePTFE of the other member through the openings in thestent. However, such a monolithically encapsulated stent tends to berather inflexible. In particular, radial expansion of the stent maystress and tear the ePTFE. There is a continuing need for a stent thatis encapsulated to prevent cellular intrusion and to provide a smoothinner surface blood flow and yet still capable of expansion withouttearing or delaminating and is relatively more flexible.

BRIEF SUMMARY

The present invention is directed to encapsulated stents whereinflexibility of the stent is retained, despite encapsulation.

It is an object of this invention to provide a stent device that hasimproved flexibility, yet maintains its shape upon expansion.

It is also an object of this invention to provide a stent encapsulatedto prevent cellular infiltration wherein portions of the stent can moveduring radial expansion without stressing or tearing the encapsulatingmaterial.

These and additional objects are accomplished by an encapsulationprocess that leaves portions of the stent free to move during expansionwithout damaging the ePTFE covering. The most basic form of thisinvention is produced by placing a stent over an inner ePTFE tube (e.g.,supported on a mandrel) and then covering the outer surface of the stentwith an outer ePTFE tube into which slits have been cut. The outer ePTFEtube is then laminated to the inner ePTFE tube through openings in thestent structure to capture the stent. By selecting the size and locationof the slits, it is possible to leave critical parts of the stentunencapsulated to facilitate flexibility and expansion. Not only doesthe slit prevent capture of the underlying PTFE, it forms a focal pointfor the ePTFE to flex. A more complex form of the process is to placeover the stent an ePTFE sleeve into which apertures have been cut. This“lacey” outer sleeve leaves portions of the stent exposed for increasedflexibility and for movement of the stent portions during expansionwithout damaging the ePTFE. Although a single stent can be used, theseapproaches lend themselves to use of a plurality of individual ringstents spaced apart along an inner ePTFE tube and covered by a “lacey”ePTFE sleeve.

In the present invention, individual ring stents are partiallyencapsulated using the procedure outlined above. Preferably, ring stentsof zigzag sinusoidal structure are placed “in phase” (e.g., peaks andvalleys of one stent aligned with those of a neighboring stent) on thesurface of a tubular ePTFE graft supported by a mandrel. A sleeve ofePTFE is cut using CO₂ laser so that openings are created, resulting ina “lacey” pattern. This “lacey” sleeve is then placed over the ringstents. The resulting structure is then subjected to heat and pressureso that regions of ePTFE become laminated or fused together where thelacey sleeve contacts the tubular graft. In addition, the ends of thestent can be completely encapsulated, by known methods, to stabilize theoverall structure.

A more complete understanding of the encapsulation process will beafforded to those skilled in the art, as well as a realization ofadditional advantages and objects thereof, by a consideration of thefollowing detailed description of the preferred embodiment. Referencewill be made to the appended sheets of drawings which will first bedescribed briefly.

BRIEF SUMMARY OF THE DRAWINGS

FIG. 1 is a perspective view of a tubular ePTFE member with individualring stents arranged thereon.

FIG. 2 is a perspective view of the “lacey” sleeve of the presentinvention.

FIG. 3 is a perspective view of the sleeve in FIG. 2 placed over thestructure of FIG. 1.

FIG. 4 is a perspective view of one configuration of the slitted sleeveof the present invention with longitudinally oriented slits.

FIG. 5 is a perspective view of a second configuration of the slittedsleeve of the present invention with circumferentially oriented slits.

FIG. 6 is a perspective view of a third configuration of the slittedsleeve as it is placed over the structure in FIG. 1.

FIG. 7 is a perspective view of an alternate embodiment of the presentinvention.

DETAILED DESCRIPTION

The present invention satisfies the need for an encapsulated stentdevice to prevent restenosis that is flexible upon expansion andcontraction so that the general structural form is retained. This isaccomplished encapsulating a stent or a plurality of stent rings usingan ePTFE covering into which openings have been cut.

Referring now to the drawings, in which like reference numbers representsimilar or identical structures throughout, FIG. 1 illustrates aninitial step in constructing the partially encapsulated stent of thepresent invention. A tubular ePTFE graft 20 is placed over a mandrel forthe assembly of a device 10 (FIG. 3). A stent is then placed over thegraft 20. In a preferred embodiment, as shown in FIG. 1, a series ofzigzag sinusoidal ring stents 30 are placed over the outer surface ofthe graft 20. Alternatively, one or more stents wherein each stentcomprises more than one ring or hoop (e.g., where the rings arehelically connected) can be used. The ring stents 30 can be made of anymaterial but a preferred material is metal. The zigzag ring stents 30may be assembled “in phase” with each adjacent ring stent having peaksand valleys aligned. Alternatively, the individual stents 30 can be “outof phase” to different degrees. It will be apparent that the phaserelation of adjacent stents 30 will alter the lateral flexibility aswell as the longitudinal compressibility of the structure. The phaserelationship can be varied along the length of the device 10, therebyaltering the physical properties in different portions of the device 10.Having individual ring stents 30, as opposed to a single tubular stent,provides the advantage that the periodicity, or the number and preciseshape of the zigzags per ring, can readily be varied along the length ofthe graft to influence flexibility and stability properties of thestructure. Also, spacing of the individual stents (number of stents perunit length) as well as the phase relationship of stent to stent can bevaried to produce stent grafts with desired properties. By placing thering stents 30 over the outer surface of the tubular ePTFE graft 20, theresulting structure has an inner (luminal) surface that is completelysmooth to facilitate the flow of blood. However, there may be instanceswhere the ring stents 30 or other tubular stents are advantageouslyplaced in contact with the inner graft surface or on both the inner andouter surfaces, as one of ordinary skill in the art will readilyappreciate.

FIG. 2 shows the structure of a “lacey” graft comprising a sleeve ofePTFE 40 into which apertures have been cut. This “lacey” graft 40 isplaced over the ring stents 30 in the preferred embodiment. The “lacey”graft 40 is created by cutting openings 44 in a tubular ePTFE graft 42.The openings 44 were cut into the sleeve by a CO₂ laser, although anyother cutting technology could readily be employed. The “lacey” graft 40is slid over the ring stents 30 and the underlying tubular graft 20 toform the preferred structure 10 shown in FIG. 3. The structure 10 isthen exposed to heat and pressure, such as that caused by wrapping withPTFE tape followed by heating in an oven, thereby causing the ePTFEregions of the “lacey” graft 40 to fuse or laminate to the tubular graft20 wherever they touch each other. It should be appreciated that thecircumferential sections of ePTFE 46 that are placed over the ringstents 30 can encompass many different designs. As illustrated, a sleeve42 with openings 44 cut out is one way of accomplishing the goal offlexibility and stability. The openings 44 between the rings of ePTFE 46can be altered to control the degree of flexibility and stabilitydesired. In the preferred embodiment shown in FIG. 3, the “lacey” graft40 forms a number of circumferential sections 46, which are intended tocover a portion of the circumference of each ring stent 30, leaving theends of the zigzags uncovered. By circumferentially covering only aportion of each ring stent 30, the maximum amount of lateral flexibilityis provided.

However, circumferentially covering the individual ring stents 30without any longitudinal support would result in a structure with littlelongitudinal strength and stability that would be prone to“telescoping”. Thus, the longitudinal sections 48 that connect the ringsof ePTFE 46 are important, because the longitudinal sections 48 arecompletely laminated to the underlying graft 20 and act as“anti-compression” devices by resisting the shortening of the structure10 (the double thickness of ePTFE resists telescoping of thelongitudinal sections 48). The width of the circumferential sections 46and the longitudinal sections 48 control longitudinal strength andstability versus lateral flexibility. By adjusting these parameters,grafts can be made more or less flexible with greater or lesseranti-compression strength. In the preferred embodiment, fourlongitudinal sections 48 are formed and the ends of the structure 10 arecompletely encapsulated for greater stability. Of course, a largernumber of longitudinal sections 48 could be formed. Also thelongitudinal sections 48 may themselves zigzag or may be helicallyarranged depending on how the openings 44 are cut into the sleeve 42.Each different structure will possess different properties. Similarly,the circumferential sections 46 can have different forms and may beundulating. There is nothing to preclude a covering with a more complexpattern where circumferential sections and longitudinal sections aredifficult to discern or are even nonexistent.

A second embodiment of the present invention can be seen in FIGS. 4-6.Instead of having a “lacey” graft structure, a slitted outer sleeve isused to provide partial encapsulation of the stent, the slits providingflexibility to the structure, allowing the stent to expand and retractmore readily. In FIG. 4, four longitudinal slits 52 run the length ofthe stent, leaving 5 to 10 mm of uncut sleeve at the ends. The slits areformed at 0°, 90°, 180°, and 270°, and are oriented to pass over a peakportion of each zigzag ring stent 30 (FIG. 6). FIG. 5 showscircumferential slits 62, wherein slits are cut circumferentially aroundthe sleeve 60 at spaced intervals, preferably to coincide with a stentring. At each radial section, two slits are cut around the circumferenceat evenly spaced intervals. In a first radial section, the slits spanfrom 0° to 90° and from 180° to 270°. Each successive radial section hasa pair of slits which are offset 90° from the previous pair. Thus, asecond radial section will have slits spanning from 90° to 180° and from270° to 0°. Beside the configurations shown in FIGS. 4 and 5, a numberof other slit configurations are possible, including diagonal andsinusoidal as will be appreciated by one skilled in the art. As shown inFIG. 6, a sleeve 70 is placed over the ring stents 30 and the underlyingtubular graft 20 to form a new structure 80. The longitudinal slits 72,which are cut into sleeve 70, differ from the slits 52 shown in FIG. 4in that they do not span the length of the structure 80 and arestaggered around the circumference of the sleeve 70. Ideally, the slitsare aligned over the peaks in the zigzag ring stents 30. Once the slits72 are cut into the sleeve 70 using any of the known methods, thestructure 80 is exposed to heat and pressure, such as that caused bywrapping with PTFE tape and heating in an oven, thereby causing theePTFE regions of the slitted graft 70 to fuse or laminate to the tubulargraft 20. The slits 72 in the slitted outer sleeve 70 can be formed byusing a CO₂ laser, razor blade or any other suitable technique known inthe art. The slits enhance the flexibility of the encapsulated structureand allow radial expansion without tearing of the ePTFE. In addition, aplurality of slits help the expanded graft to grip onto the vessel wall.This is particularly important where an encapsulated stent graft isspanning a region of damaged or weakened vessel as in an aneurysm.Further, during the healing process tissues readily grow into the slitsfurther anchoring the graft to the vessel wall.

An advantage that cutting slits into an ePTFE sleeve offers is that itis somewhat easier to manufacture than is the “lacey” graft. Because nomaterial is removed the sleeve is somewhat stronger than a “laceygraft”. There are a multitude of configurations possible, includingcutting the slits in asymmetric fashion to achieve desired results, suchas using radial, longitudinal and diagonal cuts simultaneously.Moreover, a greater number of slits can be cut into a region of thestructure in which greater expansion is desired.

Although the above examples are described with the “lacey” and slittedgrafts being placed over a stent which is itself placed over a tubulargraft, this orientation can be readily reversed. That is, the “lacey” orslitted grafts can be placed on a mandrel; a stent or stents can be thenplaced over the “lacey” or slitted grafts, and a tubular graft can bethen placed over the stent or stents. This results in a structurewherein part or much of the luminal surface is provided by the outergraft, providing superior healing as only a single layer of ePTFE wouldseparate body tissues from the blood. Moreover, a structure with two“lacey” or slitted grafts is possible. As shown in FIG. 7, the openings112 in the outer graft 110 are arranged out of phase with the openings122 in the inner graft 120. Such a configuration provides a blood tightstructure wherein a majority of the final surface area of the device 100comprises a single layer separating body tissue from the circulatingblood. Also, the areas occupied by the stent(s) 30 and by overlapbetween the two grafts 110, 120 present a barrier to cellularinfiltration. The described structure has the advantage of a smallerprofile when compressed because the overall amount of PTFE is reduced.In a further embodiment, a combination of the “lacey” graft and slittedgraft could be produced.

Having thus described preferred embodiments of the partial encapsulationof stents, it will be apparent by those skilled in the art how certainadvantages of the present invention have been achieved. It should alsobe appreciated that various modifications, adaptations, and alternativeembodiments thereof may be made within the scope and spirit of thepresent invention. For example, zigzag stent rings have beenillustrated, but it should be apparent that the inventive conceptsdescribed above would be equally applicable to sinusoidal and otherstent designs. Moreover, the words used in this specification todescribe the invention and its various embodiments are to be understoodnot only in the sense of their commonly defined meanings, but to includeby special definition in this specification structure, material or actsbeyond the scope of the commonly defined meanings. Thus, if an elementcan be understood in the context of this specification as including morethan one meaning, then its use in a claim must be understood as beinggeneric to all possible meanings supported by the specification and bythe word itself. The definitions of the words or elements of thefollowing claims are, therefore, defined in this specification toinclude not only the combination of elements which are literally setforth, but all equivalent structure, material or acts for performingsubstantially the same function in substantially the same way to obtainsubstantially the same result. The described embodiments are to beconsidered illustrative rather than restrictive. The invention isfurther defined by the following claims.

1. A method of making an implantable medical device, comprising:extruding a first ePTFE tube at a first extruded diameter; extruding asecond ePTFE tube at a second extruded diameter; cutting a plurality ofslits in the first ePTFE tube at the first extruded diameter; subsequentto the cutting, positioning a radially expandable support layer betweenthe first and second ePTFE tubes so that the slits span portions of thesupport layer; and laminating the first ePTFE tube to the second ePTFEtube through openings in the support layer.
 2. The method according toclaim 1, wherein the cutting step comprises cutting the slits parallelto a longitudinal axis of the first ePTFE tube.
 3. The method accordingto claim 2, wherein the cutting step comprises cutting slits to span amajority of the length of the first ePTFE tube, leaving an uncut regionat both a proximal and distal terminal end of the first ePTFE tube. 4.The method according to claim 1, wherein the cutting step comprisescutting the slits transverse to a longitudinal axis of the first ePTFEtube.
 5. The method according to claim 1, wherein the support layercomprises a plurality of unconnected ring stents formed in a zigzagpattern of alternating peaks and valleys, the positioning stepcomprising orienting the alternating peaks and valleys of adjacent ringstents in phase with one another.
 6. The method according to claim 5,wherein the positioning step further comprises positioning each of theslits to pass over two or more ring stent peaks or valleys.
 7. Themethod according to claim 6, wherein the position step further comprisesstaggering slits circumferentially such that adjacent slits begin andend longitudinally offset from one another.
 8. The method according toclaim 1, wherein the cutting step comprises utilizing a laser to slicethrough the wall of the first ePTFE tube.
 9. The method according toclaim 1, wherein the cutting step comprises utilizing a razor blade toslice through the wall of the first ePTFE tube.
 10. The method accordingto claim 1, wherein the cutting step comprises solely of slicing throughthe wall of the first ePTFE tube to form the slits without performing asubsequent step of removing material from the first ePTFE tube.
 11. Themethod according to claim 1, wherein the positioning step furthercomprises placing the second ePTFE tube and support layer radiallyinside the first ePTFE tube.
 12. The method according to claim 11,wherein the positioning step comprises placing the second ePTFE tube ona mandrel, placing the support layer over the second ePTFE tube, andplacing the first ePTFE tube over the support layer.
 13. The methodaccording to claim 1, wherein the cutting step further comprises cuttingslits diagonally with respect to a longitudinal axis of the first ePTFEtube.